Ionic liquids (IL) are organic salts that are commonly liquid at room temperature. ILs have also been defined as molten salts having a melting point below 100° C. Recent interest in room temperature ionic liquids has increased due to unique characteristics of those compounds, such as a unique solubility, negligible vapor pressure, a wide electrochemical window and good thermal stability. Due to the negligible vapor pressure of ILs, they have been identified as environmentally friendly as they would not contribute to air pollution or to the generation of potentially toxic emissions when used as a solvent. As a result of these advantageous characteristics, ILs have potential use in a number of applications, including but not limited to synthesis, as both solvents and catalysts; energy storage, as electrolytes; extraction of radioactive materials, metals and organic liquids; polymer processing; cellulose processing; and gas separations.
The liquid nature of ionic liquids is a barrier for these materials to be used in many device based applications. Poly(ionic liquids)s (PILs) offer a unique combination of properties including, but not limited to, the ability to be processed in into films, cross-linked materials, vesicles, and gels, all while maintaining the highly desirable properties of ILs such as solubility, high conductivity, low vapor pressure and broad electrochemical and thermal stability. The unique combination of characteristics of PILs has led to interest in technologically important applications such as antimicrobial coatings, water purification, electrolytes for batteries, and gas separation membrane sensors, among others.
The most common strategy for preparing polymerizable ionic liquid (IL) monomers is to conjugate an IL moiety to a conventional polymerizable group, e.g. (meth)acrylate, styrene, or norborene, which allows the use of well-understood polymerization techniques. This approach does allow a variety of methyl and ethylene oxide spacer groups to be placed between the IL moiety and the polymer backbone, which reduces the PILs glass transition temperature (Tg), 15 and increases the ionic conductivity. However, in many cases conjugation to large polymerizable moieties significantly reduces the functional diversity of pendant groups to simple alkyl chains, and decreases the concentration of IL groups in the resulting polymer. The decreased concentration of IL groups in the resulting polymer can be addressed by the polymerization of simpler vinyl functionalized ILs, such guanidinium, pyrrolidinium, and pyridinium cations; however, synthetic diversity in those cations is difficult to achieve. Functional imidazolium monomers are easily prepared from commercially available n-vinyl imidazole, and do allow variety of functional groups to be directly prepared via quaterization. However, while additional functionalization at the 2 and 5 positions of the ring is possible, most work has focused only on the 1,4-functionalized imidazolium cations, and only alkyl substitutions are commonly encountered. In addition, the vinyl bond on imidazolium is always on the 1 position of the imidazole ring resulting in difficult polymerization reaction.